IMAP servers that do not advertise the "UTF8=APPEND" or "UTF8=ONLY"
| capability SHOULD reject an APPEND command that includes any 8-bit in
| the message headers with a "NO" response.

It should say:

IMAP servers that do not advertise the "UTF8=APPEND" or "UTF8=ONLY"
| capability SHOULD reject an APPEND command that includes any 8-bit
| character in message header fields with a "NO" response.
or:
IMAP servers that do not advertise the "UTF8=APPEND" or "UTF8=ONLY"
| capability SHOULD reject an APPEND command that includes any 8-bit
| character in a message header with a "NO" response.